Commercial geostationary satellites typically employ shaped reflector antennas to produce directivity patterns contoured to desired coverage areas. For example, commercial satellites may have reflectors designed to produce antenna pattern contours that mimic the borders of the continental United States (CONUS), Europe, or northern Africa, as projected from orbit, thereby minimizing directivity to unserved regions. Shaped reflector antennas have the advantages of using transponder power more efficiently and having significantly lower mass than other antenna technologies producing similar results, such as phased array antennas. Shaped reflectors also have excellent pattern characteristics (particularly cross-polar discrimination, sidelobe suppression, and other pattern characteristics required for regulatory compliance and inter-operator coordination), high power handling capability, simple deployability on-orbit, and proven on-orbit reliability. These shaped reflectors have continuous, fixed, and doubly-curved surfaces, typically molded with carbon composite materials.
One disadvantage with conventional shaped reflectors is that their shape cannot be altered after manufacture. Geostationary satellites are typically built to have a lifetime of 15 years or more. Over the course of a satellite's lifetime, its operator may want to change its orbital slot or coverage area. However, because shaped reflectors are fixed to a particular orbital slot and coverage area at manufacturing, a satellite that is moved to a different orbital slot and/or is re-oriented to serve a different region would not efficiently illuminate the new coverage area. Another disadvantage with conventional shaped reflectors is that it is often difficult to repair reflector surface errors or mis-shaping after manufacturing, which can cause significant cost and schedule impacts late in satellite production.
Further, satellite manufacturers may need to design antenna systems before a satellite's orbital slot has been assigned or its intended coverage area has been defined. For example, a satellite may have a 100 degree longitudinal range within which its orbital slot will be assigned. The optimal antenna configuration for a particular coverage area depends on the orbital slot since the projected contour of a region of the earth can be dramatically different in size and shape from the vantage point of differing orbital slots. So, when the actual orbital slot is unknown, it is impossible to design an optimal antenna system. When the orbital slot is yet to be determined, the satellite manufacturer may design the reflector for a mid-range position, by averaging the footprint of the two ends of the possible range, or by enveloping all possible patterns across the entire range of projected contours. In any case, the reflector would not have been optimized for the final orbital slot, leading to suboptimal performance.
In another case, a satellite may be re-tasked by the operator in response to changing market demands to an entirely different region from its initially designated deployment, with markedly different contours (for example, moving a satellite designed for CONUS to cover Africa). In that case, the operator is forced to accept partial coverage, tolerate directivity wasted on unserved areas, and coordinate potential interference issues with adjacent satellite operators.
Furthermore, shaped reflector antennas are long-lead, pacing items in the critical path of satellite manufacturing flow and must have the definition of their surfaces finalized over a year before launch, during which time the desired coverage area might change. However, no flexibility currently exists to alter the reflector surface after fabrication.
Lastly, fixed shaped reflectors cannot compensate for one-time and dynamic on-orbit effects, such as hygroscopic distortion, diurnal and seasonal thermal distortion, and various sources of mis-alignments. In addition, fixed reflectors cannot be adjusted to address deterioration in dynamic link conditions such as regional rain fading, uplink interference, and inclined orbit operations during extended satellite life.